1. Field of the Invention
The invention relates to a headbox for a machine for producing a fibrous web, in particular a paper or cardboard web from at least one fibrous stock suspension, having a feed apparatus for feeding the at least one fibrous stock suspension, having a perforated distributor pipe plate which is located adjacent downstream and has a multiplicity of channels which are arranged in rows and columns, having an intermediate channel which is located downstream of the latter, extending over the width of the headbox and which has a plurality of means which are spaced apart from one another in width direction of the headbox, for preferably adjustable/controllable dosing of fluid in partial fluid streams into the at least one fibrous stock suspension, wherein the individual means respectively comprises a plurality of dosing channels for the fluid, which respectively have one dosing channel opening and which discharge at different heights and which are connected with a common supply channel; with a downstream turbulence generator having a plurality of flow channels arranged in rows and columns, and with a headbox nozzle comprising a nozzle gap immediately adjacent to the turbulence generator.
2. Description of the Related Art
A headbox of this type is known in the field as “Dilution water headbox” and is known for example from German disclosure documents DE 44 16 898 A1, DE 44 16 899 A1 and DE 44 16 909 A1.
German disclosure document DE 44 16 898 A1 describes a headbox for a paper machine; including a feed device for the stock suspension comprising a guiding element with a plurality of channels; upstream from a guiding element a mixing chamber with several infeed devices which are distributed over the headbox for a fluid which is to be added to the stock suspension and which differs in its characteristics from the stock suspension; and with a nozzle chamber forming an outlet opening for the stock suspension, located immediately downstream from the guiding element. The feed devices for the fluid which is to be added extend essentially vertically in the mixing chamber and comprise several dosing openings located on top of each other.
German disclosure document DE 44 16 899 A1 describes a headbox for a paper machine including a feed device for the stock suspension, with a first guiding element comprising a plurality of channels, a second guiding element comprising a plurality of channels, an intermediate mixing chamber, a contiguous nozzle chamber forming an outlet opening for the stock suspension, and several feed lines distributed over the headbox for the fluid which is to be added. The majority of the channels of the guiding elements are positioned and dimensioned so that the outlet opening of the upstream guiding element is not aligned with a confluence into the downstream guiding element. The feed lines for the fluid which is to be added are equipped with dosing openings, the majority of which align respectively with a confluence into the downstream guiding element.
In addition, German disclosure document DE 44 16 909 A1 describes a headbox for a paper machine with a feed device for the stock suspension, a downstream contiguous guiding element equipped with a plurality of channels, located downstream from there a downstream mixing chamber extending across the width of the headbox with several separation walls distributed across the headbox and extending essentially in flow direction, as well as feed devices for a fluid which is to be added, distributed in cross direction of the headbox and which are equipped with dosing openings, downstream an additional guiding element equipped with a plurality of channels and adjacent to it, one nozzle chamber forming an outlet opening for the stock suspension.
The feed devices for the fluid which is to be added extend into the mixing chamber and the sectional area which is formed in the mixing chamber by the separation walls begins in the region of the dosing openings from where it extends essentially in flow direction to the second guiding element.
In the currently known processes for producing a fibrous web, in particular a paper or cardboard web from at least one fibrous stock suspension additional fluids, for example paper chemicals, especially retention agents or dewatering accelerators are added as a rule before and/or after the separator, however prior to the headbox and in any case before or at the latest in the feed apparatus. Separators or sorters, also referred to as screens by the expert, can be in the embodiment for example of a vertical separator or as a machine wire. They serve in cleaning the at least one fibrous stock suspension before it is transported on into the headbox.
In practical operation it turned out that—viewed in flow direction—areas with strong pressure losses or respectively turbulence fields with strong shear fields follow the known locations for adding the additional fluids. The effect of the additional high molecular or respectively additional polymeric fluids, preferably paper chemicals, in particular retention agents or dewatering accelerators is thereby diminished or can be diminished. In addition, process technologically important reaction times of the mediums cannot be adjusted, or cannot be adjusted to a satisfactory level, especially defined with the known systems.
It is therefore the objective of the current invention, and what is needed in the art is, to improve a headbox of the type described at the beginning in such a manner, that the aforementioned disadvantages of the current state of the art are significantly, preferably completely eliminated. The intent is for the headbox to facilitate especially an efficient and cost effective addition of the at least one additional fluid into the at least one fibrous stock suspension into which the fluid is to be added. In particular, the trilemma should be solved so that on the one hand in certain areas of the headbox, for example in the turbulence generator turbulent flows are desirable at least to a certain extent for the purpose of intensive blending of the components of the at least one fibrous stock suspension to which the at least one fluid is added. That however, on the other hand in certain other areas of the headbox, for example in the headbox nozzle no further blending and therefore not additional swirling of the components of the at least one fibrous stock suspension to which the at least one fluid is added is desirable, and thirdly, so that the effect of certain high molecular or long-chain components of the fibrous stock suspension are not reduced, particularly through turbulence and thereby caused shear fields.